Oxide superconductor and method for producing the same

ABSTRACT

An oxide superconductor film formed on a substrate includes an oxide containing at least one metal M selected from the group consisting of yttrium and lanthanoid metals, provided that cerium, praseodymium, and promethium are excluded, and barium and copper, in which the film has an average thickness of 350 nm or more, an average amount of residual carbon of 3×10 19  atoms/cc or more, and an amount of residual fluorine in a range of 5×10 17  to 1×10 19  atoms/cc, and in which, when divided the film into a plurality of regions from a surface of the film or from an interface between the film and the substrate, each region having a thickness of 10 nm, atomic ratios of copper, fluorine, oxygen and carbon between two adjacent regions are in a range of ⅕ times to 5 times.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2006-324620, filed Nov. 30, 2006,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an oxide superconductor and a methodfor producing the same.

2. Description of the Related Art

High critical-current oxide superconductor materials that have beendeveloped in recent years are expected to be usefully applied, forexample, to fusion reactors, magnetically levitated trains, particleaccelerators, magnetic resonance imaging apparatuses (MRI) and microwavefilters, and some of these technologies have been already used inpractical fields.

Major oxide superconductors are bismuth-based, yttrium-based andthallium-based superconductors. Of these materials, yttrium-basedsuperconductors have been attracting much attention as almostpractically applicable materials since they exhibit the highestsuperconducting properties in a magnetic field at a liquid nitrogentemperature and can be used for magnetic levitated trains by coolingwith liquid nitrogen.

The yttrium-based superconductor is expressed by a formulaYBa₂Cu₃O_(7-x), and has a perovskite structure. Compounds in whichyttrium is replaced with a lanthanoid group rare earth element, andmixtures thereof have been also known to exhibit superconductingproperties. Examples of the method of producing these superconductingmaterials so far used include pulsed laser deposition (PLD), liquidphase epitaxy (LPE), electron beam (EB) processing, and metal organicdeposition (MOD).

The methods of producing the superconducting material are roughlyclassified into an in situ process and an ex situ process. The in situprocess performs deposition of metals essential for producing thesuperconductor and formation of a superconductor by oxidation the metalsat once. The ex situ process independently performs deposition of metalsessential for forming the superconductor and heat treatment for formingthe superconductor. Accordingly, a precursor (calcined film) is formedin the ex situ process.

The in situ processes were paid attention as the production methods ofthe superconductor in the early stage. This is because the in situprocesses were expected to make cost lowered due to its small number ofproduction steps. However, production conditions are hard to becontrolled in the processes since all the deposition conditions must becontrolled at once, which has been revealed that good superconductorscan hardly be obtained. On the other hand, although the ex situ processwas afraid to increase the production cost, it has been possible tosubstantially reduce the production cost by developing a non-vacuummethod such as an MOD method and a TFA-MOD method to be described below.It is an advantage of the ex situ process over the in situ process todivide the heat treatment into two steps in order to facilitate controlof heat treatment.

Examples of the ex situ process include the EB processing (P. M.Mankiewich et al., Appl. Phys. Lett. 51, 1987, 1753-1755), MOD method,and TFA-MOD method (T. Araki and I. Hirabayashi, Supercond. Sci. Techol.16, 2003, R71-R94).

In the EB processing, a precursor containing metals essential for thesuperconductor is deposited by electron beam, and a Y-basedsuperconductor is then produced by applying a heat treatment (firing). Asuperconducting layer is supposed to be developed in the firing processvia a quasi-liquid phase network in the presence of fluorine. Since nocarbon is used in this method, there is no residual carbon at all in thesuperconductor obtained, and superconducting properties are not largelyimpaired. However, the EB processing involves a problem of highproduction cost.

The MOD method is a method that has been investigated in another fieldand is used in the production of the superconductor. While much efforthas been made in order to reduce harmful residual carbon in theproduction of the Y-based superconductor by the MOD method, there is noeffective method for reducing the residual carbon. Since organicsubstances in the precursor are decomposed by calcining in this method,the film obtained is also called a calcined film. The calcined filmcontains metal oxides and residual carbon and does not contain fluorineat all. The fired film also contains the metal oxides and residualcarbon.

The TFA-MOD method derived from the MOD method will be finallydescribed. A carbon expulsion scheme works during calcining by usingtrifluoroacetic acid (TFA) as a fluorine-containing compound in thismethod, and a calcined film from which most of carbon harmful forsuperconductor is expelled is readily obtained. It has been known that ahighly oriented texture in an atomic level is formed with goodreproducibility during the firing process, by forming a quasi-liquidphase network by the action of fluorine and by a chemical equilibriumreaction. Since no vacuum process is used at all from deposition tocalcining and firing to enable the production cost to be reduced,research of this method has been spread worldwide. A wire material thatenables 70A of current with a wire having a length of 100 m can beproduced with good reproducibility today. Consequently, the TFA-MODmethod is a major process in the production of the yttrium-basedsuperconductor.

However, while the TFA-MOD method has a large advantage that asuperconductor exhibiting excellent superconducting properties may beproduced with a low production cost, it is disadvantageously difficultto increase the thickness of the film. This is because reduction in thevolume of the film is as large as 87% from a gel film to a finalsuperconducting film in this process and a stress (drying stress) in thedirection parallel to the substrate surface may be applied to the filmduring reduction in the volume, cracks are produced in the film when thefilm has a certain thickness (critical thickness) or more even byapplying a moderate heat treatment. While a high-purity solution fromwhich impurities are reduced as much as possible is usually used forobtaining a superconductor having excellent superconducting properties,the critical thickness in this case is about 300 nm. For example, when asuperconductor with a thickness of 350 nm is formed on a substrate witha diameter of 2 inches, it has been confirmed that cracks with a widthof 0.1 mm or more and a length of 1 mm or more that may be readilyrecognized by visual observation are formed.

Increasing the thickness of the superconductor will be described here.In the usual MOD method, the thickness of the film is increased byrepeating the process of coating to form a gel film followed bycalcining. The reason why such a process can be used is that thecalcining is completed in a quite short period of time. However, it hasbeen known that fatal degradation in superconducting properties iscaused due to an increased amount of residual carbon when the thicknessof the superconductor is increased by repeated coatings in the MODmethod. On the other hand, in the TFA-MOD method, a gradually increasingtime sufficient for breaking covalent bonds is necessary in thecalcining step for decomposing organic substances by calcining while theorganic substance is prevented from being burnt, and the longest time isspent in the entire process for calcining. Accordingly, a quite longtime of heat treatment is necessary for increasing the thickness of thesuperconductor when coating is repeated in the TFA-MOD method.Homogeneity of the superconducting film is lost due to localcrystallization when the film experiences many times of heat hysteresis,and the quality of the film is gradually degraded. In addition, it hasbeen known that superconducting properties are degraded by repeatedcoatings since an oxide layer is formed at the boundary between thelower layer and upper layer formed by repeated coatings. In particular,increasing the thickness by repeated coatings causes a critical effectin the application of the superconductor for a microwave filter.Usually, a superconductor with a thickness of 400 nm or more is assumedto be necessary for the microwave filter, and a thick superconductorthat maintains excellent superconducting properties is necessaryparticularly in a signal transmission side. However, since intermediateoxide layers are formed by repeated coatings as described above,adjustment of filter characteristics becomes quite difficult due to theoxide as a cause of loss to make it difficult to produce a sharp-cutfilter. Accordingly, it is important to obtain a thick superconductorthat exhibits excellent superconducting properties by single coating inthe TFA-MOD method, in order to prevent superconducting properties frombeing degraded by long term heat treatment and by formation of oxidelayers.

Next, a method for thickening the superconductor by single coating willbe described below. In the usual MOD method, an organic compound havinga longer chain is added to a metal organic compound containing essentialmetals for forming the superconductor. In this case, the film isprevented for cracks from being produced by taking advantage of the factthat the added organic compound is not decomposed at a low temperaturefor decomposing the metal organic compound that contains the essentialmetals. The added organic compound is decomposed thereafter at a highertemperature. The film thickness may be practically increased by usingsuch a method. However, residual carbon originating from the addedorganic compound is a problem in the MOD method. On the other hand, ithas been found in recent years that, when the same method is used in theTFA-MOD method, residual fluorine is problematic rather than residualcarbon by virtue of the action of the carbon expulsion scheme. While acertain extent of fluorine is necessary in the TFA-MOD method forforming a quasi-liquid phase network in the firing process, the contentof residual fluorine accounts for about 10 to 20 times when the sameorganic compounds as used in the MOD method are added as compared withthe case when no such organic compound is added. It has been alsoanticipated that fluorine may be eliminated by keeping the firingcondition for a long term since fluorine is eliminated when thequasi-liquid phase network is formed. However, it has been found that aminute amount of a texture of a Y-based superconductor in which fluorinecompounds are mixed is formed by keeping the superconductor under thefiring condition for a long period of time. The fluorine compound isrecrystallized into barium fluoride during cooling, which leads todisturbance of crystal orientation. Actually, a superconductor hassuperconducting properties 1/10 or less of the original level when justa minute amount of BaF₂ is detected by XRD measurement of thesuperconductor. Further, when the organic compound is added, the amountof residual fluorine increases by increasing the thickness of the filmformed by single coating. This may be believed that fluorine existing atthe bottom of the coating film is not eliminated.

Rupich et al have proposed a method for preventing the fluorine contentin the oxide superconductor from increasing by using Cu carboxylatecontaining less fluorine in place of Cu trifluoroacetate without addingany fluorine-containing organic compounds (WO 2002/035615). Cucarboxylate used in this method contains, for example, chlorine, bromineor hydrogen in place of fluorine. However, since the solution used byRupich et al is not a high-purity solution prepared by aSolvent-Intro-Gel method, it may be supposed that a certain amount ofacetate salt remains already, which possibly increases the amount ofresidual fluorine.

While J. A. Smith et al have reported a superconductor having athickness of as large as 1,000 nm, superconducting properties are not sohigh and the details of the production method are indefinite.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided anoxide superconductor film formed on a substrate, comprising: an oxidecontaining at least one metal M selected from the group consisting ofyttrium and lanthanoid metals, provided that cerium, praseodymium, andpromethium are excluded, and barium and copper, wherein the film has anaverage thickness of 350 nm or more, an average amount of residualcarbon of 3×10¹⁹ atoms/cc or more, and an amount of residual fluorine ina range of 5×10¹⁷ to 1×10¹⁹ atoms/cc, and wherein, when divided the filminto a plurality of regions from a surface of the film or from aninterface between the film and the substrate, each region having athickness of 10 nm, atomic ratios of copper, fluorine, oxygen and carbonbetween two adjacent regions are in a range of ⅕ times to 5 times.

According to another aspect of the present invention, there is provideda method for producing an oxide superconductor, comprising: preparing acoating solution by mixing a methanol solution in which correspondingfluorocarboxylate salts are contained so as to set an atomic ratio ofmetal M selected from the group consisting of yttrium and lanthanoidmetals, provided that cerium, praseodymium, and promethium are excluded,and barium and copper to be approximately 1:2:3 with an organicsubstance having a ratio of fluorine/(fluorine and hydrogen) in a rangeof 75 to 96 mol %; forming a gel film by coating the coating solution ona substrate; and forming a film of an oxide superconductor by calcining,firing and oxygen-annealing the gel film.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a flowchart for preparing a high-purity solution according toan embodiment;

FIG. 2 is a flowchart for producing a superconductor according to anembodiment;

FIG. 3 is a graph showing a temperature profile in a calcining processaccording to an embodiment;

FIG. 4 is a graph showing a temperature profile in a firing processaccording to an embodiment;

FIG. 5 is a graph showing a SIMS profile of a calcined film formed onthe front surface of a substrate in Example 2;

FIG. 6 is a graph showing a SIMS profile of a calcined film formed onthe back surface of the substrate in Example 2;

FIG. 7 is a graph showing a SIMS profile of a superconducting filmformed on the front surface of the substrate in Example 2;

FIG. 8 is a graph showing a SIMS profile of a superconducting filmformed on the back surface of the substrate in Example 2;

FIG. 9 is a graph showing a SIMS profile of a calcined film formed inExample 3;

FIG. 10 is a graph showing a relation between R_(F) and Jc of asuperconducting film obtained in Example 5; and

FIG. 11 is a graph showing the changes of fluorine concentrationdepending on additives in the superconducting film obtained in Example8.

FIG. 12 is a graph showing the compositional distribution measured bySIMS analysis in the direction from the surface to the substrate withrespect to superconducting film F0709.

FIG. 13 is a graph showing the compositional distribution measured bySIMS analysis in the direction from the surface to the substrate withrespect to superconducting film F0809.

FIG. 14 is a graph showing the compositional distribution measured bySIMS analysis in the direction from the surface to the substrate withrespect to superconducting film F0910.

FIG. 15 is a graph showing the compositional distribution measured bySIMS analysis in the direction from the surface to the substrate withrespect to superconducting film F0911.

FIG. 16 is a graph showing the compositional distribution measured bySIMS analysis in the direction from the surface to the substrate withrespect to superconducting film F0913.

FIG. 17 is a graph showing the compositional distribution measured bySIMS analysis in the direction from the surface to the substrate withrespect to superconducting film F0915.

FIG. 18 is a graph showing the compositional distribution measured bySIMS analysis in the direction from the surface to the substrate withrespect to superconducting film F1118.

FIG. 19 is a graph showing the compositional distribution measured bySIMS analysis in the direction from the surface to the substrate withrespect to superconducting film F1119.

FIG. 20 is a graph showing the compositional distribution measured bySIMS analysis in the direction from the surface to the substrate withrespect to superconducting film F1120.

FIG. 21 is a graph showing the compositional distribution measured bySIMS analysis in the direction from the surface to the substrate withrespect to superconducting film F0918.

FIG. 22 is a graph showing the compositional distribution measured bySIMS analysis in the direction from the surface to the substrate withrespect to superconducting film F1318.

DETAILED DESCRIPTION OF THE INVENTION

In the method of producing an oxide superconductor according to anembodiment of the invention, a coating solution used is a methanolsolution of a fluorocarboxylic acid salt containing metal M, barium andcopper to which an organic compound with a fluorine/(fluorine andhydrogen) ratio in the range of 75 to 96 mol % is added. So far, therehave been no attempts to add an organic compound containing a largeamount of fluorine to the coating solution. This is because the amountof residual fluorine is supposed to increase by this method.

The present inventors have elucidated that many phenomena in the TFA-MODmethod may be interpreted by a carbon expulsion scheme in the calciningprocess and by formation of a texture based on a quasi-liquid phasenetwork model in the firing process. For taking advantage of these twomechanisms, a long chain fluorocarboxylic acid has been suggested as asubstance that is able to suppress the amount of residual fluorine fromincreasing in the superconductor while esterification reactions insolution preparing processes are prevented.

An outline of the reason why the long chain fluorocarboxylic acid isadvantageously used is as follows. When an organic compound containingmany atoms other than fluorine such as hydrogen is used, decomposedfluorine compounds remain within the film up to a higher temperaturesince the decomposed fluorine compounds form hydrogen bonds with theorganic compound. Since fluorine replaces oxygen according to the carbonexpulsion scheme, the amount of residual fluorine increases when a lotof fluorine compounds remain for a longer period of time. Accordingly,it is important that the fluorine compounds formed in the calciningprocess do not form hydrogen bonds in order to prevent increase inresidual fluorine.

While atoms having electronegativity close to that of fluorine may beexpected to cause no interaction with fluorine, there are no other atomshaving electronegativity close to that of fluorine having the strongestelectronegativity among all the atoms. However, it may be supposed thatinteraction with the fluorine compound formed in the calcining processmay be excluded by using an additive containing fluorine itself. When along chain fluorocarboxylic acid is added as the additive, that additiveis decomposed at 200 to 250° C. that is a retention temperature for thecalcining process, while the decomposed product does not stronglyinteract with surrounding fluorine compounds originating fromtrifluoroacetic acid by forming, for example, hydrogen bonds. Theadditives decomposed by calcining are reduced into small molecularweight compounds, and are gasified and dissipated into air stream. It iseffective for reducing the amount of residual fluorine in the finallyobtained superconductor to add, as the additive, an organic compoundthat contains a large amount of fluorine and few or no hydrogen.However, all the atoms bonded in an organic chain are not necessarilyfluorine, and there is no problem when hydrogen is bonded to a part ofthe chain. Small amount of hydrogen surrounded by a large amount offluorine causes no problem since hydrogen is dissipated by forminghydrogen fluoride. It has been disclosed in this invention that thenumber of the hydrogen atoms ⅓ or less of the number of fluorine atomsin the additive, or a proportion of F/(F+H) of 75 mol % or more, iseffective for reducing the amount of residual fluorine. A thicksuperconducting film that contains a small amount of residual fluorineand exhibits excellent superconducting properties may be obtained byadding a fluorine compound to a high-purity coating solution as theadditive.

Since the long chain fluorocarboxylic acid added as the additive isdecomposed at a temperature different from the decomposition temperatureof trifluoroacetic acid, cracks may be prevented from emerging in thecalcined film. Fluorine compounds formed by cleaving organic chains aredissipated in air stream as a gas without interacting with othersubstances. Consequently, since oxygen bonded to metals constituting thesuperconductor is not replaced with fluorine, the amount of residualfluorine in the superconducting film may be specifically reduced toobtain a superconducting film that exhibits excellent superconductingproperties.

The amount of the additive added to the coating solution, which isrevealed to be effective for increasing the thickness, is in the rangeof 2.5 to 20 wt % for a standard solution having a molar concentrationof metal ions of ⅕ mol/L. It has been shown from detailed investigationsof various additives that the amount of the additive is related to aweight ratio, not to the molar ratio. It has been also known that theaddition amount of the additive may be increased in proportion to theconcentration of metal ions when the metal ion concentration in thecoating solution is different. For example, when a solution having amolar concentration of the metal ions of 2.1 mol/L is used, which is 40%higher than the concentration in the above standard solution, theaddition amount of the additive may be also 40% higher than theabove-mentioned range. This shows that the proportion of the solute andadditive that constitute the gel film may fall within a specified ratioin the gel film after coating. The weight of the solute is approximatedby the weight of the solution, because the volume decreases whenmethanol is mixed with trifluoroacetate salt as the solute and anaccurate weight of the solute per unit volume cannot be determined.

The reason why the effect of disregarding the weight of methanol is notso large may be appreciated from the following reason. The upper limitof the metal ion concentration in a mixed trifluoroacetate salt is fromabout 2.9 to about 3.0 mol/L. However, since using a high concentrationof solution tends to increase the stress of the gel film, the upperlimit of the practically available metal ion concentration is in therange of about 2.7 to about 2.8 mol/L. This concentration is less thantwice of the metal ion concentration of 1.5 mol/L of the standardsolution. On the other hand, the lower limit of the metal ionconcentration of mixed trifluoroacetate salt is 0.75 ml/L, since a thickgel film is hardly obtained even under an optimum coating condition whenthe concentration is lower than 0.75 ml/L. Thus, the lower limit of themetal ion concentration is about 0.5 times the concentration of thestandard solution of 1.5 mol/L. Accordingly, the effective additionamount may be roughly estimated by approximating the addition amount ofthe additive as a ratio to the weight of the solution. Since thespecific gravity of the mixed trifluoroacetate salt containing yttriumis about 2.4 g/ml while the specific gravity of methanol is 0.79 g/ml,the contribution of methanol that has a light weight in the solution issmall. This may be also a reason why the effect of disregarding theweight of methanol is negligible.

Since the critical thickness is about 350 nm when the addition amount ofthe additive to the coating solution having a standard concentration is2.5 wt %, the thickness of the superconducting film cannot be increasedat an addition amount less than the above-mentioned value. Most of theadditives can contribute to form a precursor (calcined film) of asuperconducting film with a thickness of as large as about 3,000 nm at aconcentration of about 10 wt %. Good superconducting properties arehardly obtained when the addition amount of the additive exceeds 20 wt %since the proportion of the additive in the film increases.

A calcined film with a thickness of 700 nm or more and a fired film(superconducting film) with a thickness of 350 nm or more may beobtained by single coating without incorporation of an oxide layer byusing the method according to an embodiment of the invention. Inparticular, excellent superconducting properties are obtained in thefired film (superconducting film) with a thickness of 650 nm obtainedfrom a calcined film with a thickness of 1,300 nm. A calcined film witha thickness of 2,900 nm (corresponding to 1,450 nm after firing) havingno discontinuous planes for the distribution of the composition is alsoobtained. The oxide superconductor film preferably has an averagethickness of 600 nm or more. The oxide superconductor film morepreferably has an average thickness of 1,250 or more. Whilesuperconducting properties are not obtained in a thick film due toemergence of a lot of a/b-axis oriented grains since the film isdeposited on LaAlO₃ at present, a superconducting film having excellentsuperconducting properties may be expected by forming the film on anintermediate layer on which grains oriented in a/b axis are hardlyformed in the firing process.

The method for producing the oxide superconductor according to anembodiment of the invention will be described in more detail.

The method for preparing a high-purity solution by allowing the metalacetate salt to react with fluorocarboxylic acid will be described withreference to FIG. 1. The metal acetate salt (a1) in FIG. 1 is acollective name of acetate salt of metal M, barium acetate and copperacetate. Each metal acetate salt (a1) is dissolved in water (b), andmixed with fluorocarboxylic acid (a2). These solutions are allowed toreact by mixing so that the proportion of metal ions is 1:2:3 in molarratio (c), the mixed solution is purified (d) by allowing impurities toevaporate in vacuum, and a powder or gel (e) containing impurities isobtained.

While a fluorocarboxylic acid having two carbon atoms, for exampletrifluoroacetic acid (TFA), is used as the fluorocarboxylic acid (a2),other appropriate fluorocarboxylic acids may be used depending on thekind of the metal acetate salt. For example, a metal acetate saltcontaining a metal M and copper acetate may be allowed to react with thefluorocarboxylic acid having two carbon atoms as well as with afluorocarboxylic acid having three or more carbon atoms, for examplepentafluoropropionic acid. In particular, a metal acetate saltcontaining a metal M selected from the group consisting of lanthanum,neodymium and samarium is preferably made to react with afluorocarboxylic acid having three or more carbon atoms. However, sincebarium acetate forms a precipitate by reacting with the fluorocarboxylicacid having three or more carbon atoms, it is made to react with thefluorocarboxylic acid having two carbon atoms, for example TFA.

Examples of the fluorocarboxylic acid having two carbon atoms includetrifluoroacetic acid (TFA), monofluoroacetic acid (MFA) anddifluoroacetic acid (DFA). Examples of the fluorocarboxylic acid havingthree or more carbon atoms include pentafluoropropionic acid (PFP),heptafluorobutanoic acid (HFB) and nonafluoropentanoic acid (NFP).

The gel or powder is purified thereafter by Solvent-Into-Gel (SIG)method. Specifically, an impurity (for example water) is replaced withmethanol by adding methanol to the powder or gel (e) containingimpurities, the powder or gel is purified (h) by permitting methanol andimpurity to dissipate from the solution (g) containing the impurities,and a powder or gel containing a solvent (i) is obtained. A high-puritysolution (k) is obtained by adding methanol (j) to the powder or gel (i)containing the solvent again.

The method for obtaining an oxide superconductor will be described belowwith reference to FIG. 2. Solution A is prepared by mixing respectivehigh-purity solutions so that the molar ratio of metal M, barium andcopper is 1:2:3, and coating solution B is prepared (b) by adding anorganic compound (a) with a fluorine/(fluorine+hydrogen) ratio in therange of 75 to 96 mol % as an additive. A gel film (d) is formed bydeposition (c) of the coating solution (B) on a substrate, a calcinedfilm (f) containing a metal oxy-fluoride is formed by calcining (e) as aprimary heat treatment, and an oxide superconductor (i) is obtained byapplying firing (g) as a secondary heat treatment and annealing (h) inpure oxygen.

FIG. 3 shows an example of the temperature profile (and atmosphere) inthe calcining process.

(1) The temperature in the heat treatment furnace is rapidly increasedfrom room temperature to 100° C. within a period from time 0 to t_(a1)(about 7 minutes from the start of heat treatment). The inside of theheat treatment furnace is under a dry oxygen atmosphere at an ambientpressure in this step. All the heat treatment steps thereafter may beperformed under an atmospheric pressure.

(2) The atmosphere in the heat treatment furnace is changed to a moistpure oxygen atmosphere of an ambient pressure at time t_(a1). Thetemperature in the heat treatment furnace is increased from 100° C. to atemperature range from 170 to 230° C. during the interval from timet_(a1) to t_(a2) (about 42 minutes from the start of the heattreatment). The relative humidity of the moist pure oxygen atmosphere isset, for example, in the range of 1.2 to 12.1%. The above-mentionedhumidity corresponds to a dew point of 10° C. and 50° C., respectively.The humidity is controlled by the saturated water vapor pressure inbubbles by passing the atmosphere gas (oxygen gas) through water at apredetermined temperature, i.e. the saturated water vapor pressure isdetermined by the temperature. For setting the dew point temperature ofthe corresponding humidity to be lower than room temperature, theatmosphere gas flow is split to permit only a part of bubbles of theatmosphere gas to pass through water followed by allowing the split gasstream to join the main gas stream. The object of moistening is toprevent copper fluoroacetate from being sublimed by converting copperfluoroacetate that is most readily sublimed into an oligomer by partialhydrolysis in order to increase the apparent molecular weight of copperfluoroacetate. When fluoroacetic acid is trifluoroacetic acid, in thehydrolysis reaction that proceeds as follows, a hydrogen bond is formedbetween the F atom and H atom at both terminals of the copper salts, andsublimation is suppressed by increasing the apparent molecular weight bylinking 4 to 5 molecules.CF₃COO—Cu—OCOCF₃+H₂O→CF₃COO—Cu—OH+CF₃COOH

(3) The temperature in the furnace is slowly increased to the range of220 to 280° C. in an interval from t_(a2) to t_(a3) (from about 4 hoursand 10 minutes to about 16 hours and 40 minutes). The temperature isslowly increased in order to prevent carbon components from being leftbehind due to combustion of the partially hydrolyzed salt as a result ofabrupt reaction. Covalent bonds in the salt are cleaved by a long termdecomposition reaction, metal oxides (Y₂O₃, BaO, CuO) are temporarilyformed, a part of oxygen is replaced with F in Y₂O₃ and BaO, andnon-stoichiometric compounds of Y or Ba with oxygen and fluorine areformed. When the reaction slowly proceeds while the temperature isgradually increased, only grains of CuO as a single substance are growninto nano crystalline crystals with a diameter of several tens ofnanometers, while Y and Ba components having a non-stoichiometric ratiobetween fluorine and oxygen remain amorphous without grain growth.

(4) The temperature in the heat treatment furnace is increased from therange of 220 to 280° C. to 400° C. from time t_(a3) to t_(a4) and fromtime t_(a4) to t_(a5) (a time interval of about 2 hours, respectively).While unnecessary organic substances formed by decomposition during timet_(a2) to time t_(a3) are left behind by forming hydrogen bonds, theyare eliminated in this step.

(5) The furnace is cooled by flowing a gas at time t_(a5) andthereafter. A calcined film is thus obtained.

The calcined film obtained is fired and annealed in pure oxygen in theelectric furnace to produce an oxide superconductor. An example of thetemperature profile (and atmosphere) during the firing process is shownin FIG. 4.

(6) The temperature in the heat treatment furnace is rapidly increasedfrom room temperature to 100° C. during time 0 to t_(b1) (about 7minutes from the start of the heat treatment). The inside of the heattreatment furnace is kept under an Ar/O₂ gas atmosphere at an ambientpressure. The oxygen concentration is selected to be an optimumconcentration depending on the metal species of the superconductor to befired and firing temperature. For example, when a Y-based material(YBa₂Cu₃O_(7-x)) is fired at 800° C., the optimum heat treatmentcondition is to set an initial oxygen partial pressure at 1000 ppm, andthe oxygen concentration is approximately halved for every temperaturedecrease of 25° C. While the oxygen concentration is also approximatelyhalved for every temperature decrease of 25° C. in La-based, Nd-basedand Sm-based materials, the partial pressure of oxygen for firing at800° C. is preferably 1 ppm, 5 ppm and 20 ppm, respectively. All theheat treatment steps thereafter may be applied under an ambientpressure.

(7) The temperature in the heat treatment furnace is increased to ahighest temperature of 750° C. to 825° C. during time t_(b1) to timet_(b2) (about 33 minutes to 37 minutes, heated at a heating rate ofabout 20° C./min until the highest temperature is reached) and duringtime t_(b2) to time t_(b3) (about 5 minutes). The dry gas is moistenedat time t_(b1) by the same method as in calcining. The moistening ratioin this step may be selected in a wide range of 1.2% (dew point 10° C.)to 30.7% (dew point 70° C.). The reaction rate increases by increasingthe moistening ratio. The rate of increase is estimated to beproportional to 0.5 power of moistening ratio. The heating rate duringtime t_(b2) to time t_(b3) is small in order to suppress overheating ofthe electric furnace at time t_(b3). A quasi-liquid phase begins to formby the action of water vapor at a temperature of about 650° C., and anetwork of the quasi-liquid phase is formed in the film.

(8) MBa₂Cu₃O₆ is sequentially formed on the substrate from thequasi-liquid phase network during time t_(b3) to time t_(b4) (from about45 minutes to about 3 hours and 40 minutes; this time interval dependson the highest temperature and final thickness, and is the longest whenthe temperature is low and the thickness of the film is large) withsimultaneous emission of HF gas. The chemical reaction of this step maybe simply represented as:(M-O—F:amorphous)+H₂O→M₂O₃+HF↑(Ba—O—F:amorphous)+H₂O→BaO+HF↑(½)M₂O₃+2BaO+3CuO→MBa₂Cu₃O₆

(9) The atmosphere is switched to dry Ar/O₂ gas from time t_(b4). Thereason for switching the atmosphere to the dry gas is that the oxideMBa₂Cu₃O₆ formed before time t_(b4) is decomposed with water vapor atnear 600° C., although the oxide is stable to water vapor at near 800°C.

(10) The temperature in the heat treatment furnace is continuouslydecreased from time t_(b5) to time t_(b6) (from 2 hours to 3 hours and30 minutes) following the time t_(b4) to time t_(b5) (about 10 minutes).No changes occur in the oxide formed during this time period.

(11) The atmosphere is switched from dry Ar/O₂ gas to dry pure oxygengas at time t_(b6). MBa₂Cu₃O₆ is converted into MBa₂Cu₃O_(7-x) (x isabout 0.07) by annealing in pure oxygen, and an oxide superconductor isobtained. The temperature for switching to pure oxygen differs dependingon the metal M. It is known that a good oxide superconductor may beobtained at 525° C. when M is Y, in a range form 425 to 525° C. when Mis Sm, in the range of 375 to 475° C. when M is Nd, and in the range of325 to 425° C. when M is La.

EXAMPLES Example 1

Powders of hydrates of Y(OCOCH₃)₃, Ba(OCOCH₃)₂ and Cu(OCOCH₃)₂ weredissolved in ion-exchange water, and each solution was mixed with anequimolar amount of CF₃COOH for reaction with stirring. These solutionswere mixed so that molar ratio of metal ions is 1:2:3 to obtain a mixedsolution. The mixed solution obtained was placed in a round-bottomflask, and was allowed to react for 12 hours in vacuum with purificationusing a rotary evaporator to obtain a semi-transparent blue gel or sol.

A solution, which was obtained by completely dissolving the gel or solby adding methanol (FIG. 1 f) corresponding to 100 times the weight ofthe gel or sol, was purified in vacuum for 12 hours using a rotaryevaporator to obtain a semi-transparent blue gel or sol.

The gel or sol obtained was dissolved again in methanol (FIG. 1 j), andwas diluted using a measuring flask to obtain coating solution A with aconcentration of 1.52M as reduced into the concentration of metal ions.

Coating solution B was obtained by adding 10 wt % of H(CF₂)₈COOH to thecoating solution A as an additive.

The coating solution A and coating solution B were placed in respective100 cc beakers at a depth of about 30 mm. An oriented LaAlO₃ singlecrystal substrate with both surfaces polished was dipped into eachbeaker, and the substrate was withdrawn with a withdrawal speed of 5mm/sec or 20 mm/sec. Four kinds of gel films were thus obtained. Anindex “a” is attached to the gel film obtained from the coating solutionA, while an index “b” is attached to the gel film obtained from thecoating solution B. An index “w05” is attached to the gel film obtainedat a withdrawal speed of 5 mm/sec, while an index “w20” is attached tothe gel film obtained at a withdrawal speed of 20 mm/sec. Accordingly,four kinds of the gel films obtained are referred to as G1aw05, G1aw20,G1bw05 and G1bw20, respectively.

The Gel films G1aw05, G1aw20, G1bw05 and G1bw20 were placed inrespective calcining furnaces, and organic substances were decomposed byheat treatment under a moist oxygen atmosphere according to thetemperature profile shown in FIG. 3 to obtain calcined films containingsemi-transparent dark brown metal oxy-fluoride. These calcined films arereferred to as C1aw05, C1aw20, C1bw05 and C1bw20, respectively. Thesecalcined films had a thickness of about 400, 800, 400, and 800 nm,respectively.

While no cracks were appeared in the calcined film C1aw05 obtained formthe coating solution A, cracks capable of being visually observed with awidth of 0.1 mm or more and a length of 1 mm or more were produced onthe calcined film C1aw20 obtained form the coating solution A. It isknown that the critical thickness of the calcined film is about 600 nmthat is about twice as large as the critical thickness of the fired film(about 300 nm). The result of C1aw20 supports that the film with athickness of 800 nm that exceeds the critical thickness of the calcinedfilm of 600 nm tends to produce cracks.

On the other hand, no cracks were produced in the calcined films C1bw05and C1bw20 obtained from the coating solution B. This means that crackswere prevented from being produced by the organic substances added. Novisually observable cracks with a width of 0.1 mm or more and a lengthof 1 mm or more were produced at all on these calcined films in a regionexcept a region within 2 mm from the edge of the substrate. It may beexpected that no cracks would be produced even by depositing on a largerarea of substrate as long as no cracks are produced in theabove-mentioned area. This is because the width of 6 mm is sufficientlylarge in the length and ratio relative to the thickness of 800 nm, andstress caused by contraction in the direction of the surface of thesubstrate may be relaxed in all regions. Accordingly, excellentsuperconducting properties may be expected by using the invention. Inother words, good calcined films without any cracks are obtained on bothsurfaces of the substrate by adding an organic substance having a highfluorine content even when the films having a thickness greater than thecritical thickness are deposited on both surfaces of the substrate usinga high-purity solution.

Three calcined films of C1aw05, C1bw05 and C1bw20 except C1aw20 thatproduced cracks were placed in a firing furnace, and superconductorswere obtained by applying firing according to the temperature profileshown in FIG. 4 and by oxygen-annealing.

Most cracks are formed on the film in the calcining process in theTFA-MOD method, and heat treatment proceeds by maintaining theconfiguration of the calcined film in the firing process. There were nofilms that produced cracks during the firing process in this example.These superconducting films (fired films) are referred to as F1aw05,F1bw05 and F1bw20.

The superconducting properties of the superconducting film obtained weremeasured by an inductive method in a self magnetic field in liquidnitrogen with CryoScan (manufactured by THEVA Co.). The thickness of thefilm was measured by destructive analysis by induced coupled plasmaemission spectrometry (ICP) after the measurement by the inductivemethod.

The superconducting film F1aw05 obtained from the coating solution A hada thickness of 200 nm, and a Jc value was as high as 7.34 MA/cm² (77K,0T).

The superconducting film F1bw05 obtained from the coating solution B hada thickness of 230 nm with the Jc value of 6.58 MA/cm² (77K, 0T), anddegradation of the superconducting properties was insignificant even byadding the additive.

The superconducting film F1bw20 obtained from the coating solution B hada thickness of 500 nm with the Jc value of 5.26 MA/cm² (77K, 0T). It wasshown that a superconductor showing an electric current of 263 A per 1cm width could be obtained with the thickness formed by single coatingas described above.

Example 2

Powders of hydrates of Y(OCOCH₃)₃, Ba(OCOCH₃)₂ and Cu(OCOCH₃)₂ weredissolved in ion-exchange water, and each solution was mixed with anequimolar amount of CF₃COOH for reaction with stirring. These solutionswere mixed so that the molar ratio of metal ions was 1:2:3 to obtain amixed solution. The mixed solution obtained was placed in a round-bottomflask, and was allowed to react for 12 hours in vacuum with purificationusing a rotary evaporator to obtain a semi-transparent blue gel or sol.

A solution, which was obtained by completely dissolving the gel or solby adding methanol (FIG. 1 f) corresponding to 100 times the weight ofthe gel or sol, was purified in vacuum for 12 hours using a rotaryevaporator to obtain a semi-transparent blue gel or sol.

The gel or sol obtained was dissolved again in methanol (FIG. 1 j), andwas diluted using a measuring flask to obtain coating solution 2A with aconcentration of 1.52M as reduced into the concentration of metal ions.

Coating solution 2B was obtained by adding H(CF₂)₈COOH to the coatingsolution 2A in a concentration of 10 wt % as an additive. The coatingsolution 2B was placed in a 100 cc beaker so that the depth of thesolution was about 30 mm, an oriented LaAlO₃ single crystal substratewith both-side polished was dipped into each beaker, and the substratewas withdrawn with a withdrawal speed of 50 mm/sec on one minute afterthe dipping. Two samples of double-sided gel films G2b1w50 and G2b2w50were obtained under the same condition as described above.

Each of the gel films G2b1w50 and G2b2w50 was placed in a calciningfurnace, and organic substances were decomposed by heating in a moistoxygen atmosphere according to the temperature profile shown in FIG. 3to obtain a calcined film made of a semi-transparent dark brown metaloxy-fluoride. These calcined films are referred to as C2b1w50 andC2b2w50.

FIGS. 5 and 6 show the results of compositional distribution in thedirection from the surface to the substrate on the front surface andback face of C2b1w50, respectively, determined by SIMS analysis. Areference film was independently prepared by ion-implantation offluorine and carbon, and fluorine and carbon in C2b1w50 were quantifiedwith reference to the reference film. FIGS. 5 and 6 show that thedistribution curves have similar tendency on the front surface and backsurface.

The zero position on the horizontal axis denotes the surface of the filmin FIGS. 5 and 6. The content of carbon is higher on the surface of thefilm than the inside of the film, because carbon dioxide in the air isadsorbed on the surface of the film. Since the surface of thesuperconducting film formed by the TFA-MOD method is rough, the effectof adsorption of CO₂ in the air extends to a depth of about 200 nm fromthe surface. It is also observed that the content of carbon is reducedby the order of 1.5 figures at a depth of 1.38 μm or more that isconsidered as an interface between the calcined film and single crystalsubstrate. This is due to a minute amount of remaining carbon componentin the film originating from trifluoroacetic acid as a starting materialof the TFA-MOD method. Such residual carbon is not observed in theprecursor of the EB method.

With respect to the distribution of oxygen, fluorine and copper in FIGS.5 and 6, it may be confirmed that analyzed values of these elements arequite stable in the region from a depth of 200 nm in the vicinity of thesurface to a depth of 1240 nm where a portion corresponding to 10% ofthe thickness of the film is excluded from the interface. No abruptchange of the composition, which is specific to the film formed byrepeated coatings and exceeds 5 times or more the concentration in theintermediate oxide layer, are observed in these drawings. This confirmsthat a uniform film can be formed by single coating.

The calcined film C2b2w50 was placed in a firing furnace, and asuperconductor F2b2w50 was obtained by firing the calcined filmaccording to the temperature profile shown in FIG. 4 followed byoxygen-annealing.

Superconducting properties of the superconductor obtained were measuredby an inductive method in a self-magnetic field in liquid nitrogen withCryoScan (manufactured by THEVA Co.). The Jc values of F2b2w50 were 5.11MA/cm² (77K, 0T) in the superconducting film on the front surface and5.64 MA/cm² (77K, 0T) on the back surface.

FIGS. 7 and 8 show the results of measurement of the compositiondistribution in the direction from the surface to the substrate of thefront surface and back surface, respectively, of F2b2w50 by SIMSanalysis. FIGS. 7 and 8 show only the intensities since, unlike in FIGS.5 and 6, no comparison is made based on a reference film prepared byion-implantation of fluorine and carbon. The intensity is not alwaysproportional to the amount of substances in the SIMS analysis.

FIG. 7 shows that the thickness of the superconducting film on thesurface is about 660 nm, and no discontinuous planes are formed withrespect to oxygen, fluorine and copper as in the calcined film. In otherwords, discontinuous changes of the amounts of components, which areobserved in the superconducting film obtained by repeated coatings, arenot observed in this superconducting film.

The thickness of the superconducting film on the back surface is about600 nm, which is smaller than the thickness on the front surface. Thisdifference corresponds to a difference of thickness in the range ofabout ±5% of the film formed by dip coating. No discontinuous changes ofthe amounts of components are observed in the superconducting filmformed on the back surface of the substrate as in that on the frontsurface of the substrate.

As hitherto described, a superconductor with a thickness of more than400 nm and having no discontinuous planes, which can be used as asuperconducting filter, was obtained while superconducting properties of5 MA/cm² (77K, 0T) or more was maintained. Such superconductingproperties have been considered to be obtained from only a high-puritysolution.

Example 3

Powders of hydrates of Y(OCOCH₃)₃, Ba(OCOCH₃)₂ and Cu(OCOCH₃)₂ weredissolved in ion-exchange water, and each solution was mixed with anequimolar amount of CF₃COOH for reaction with stirring. These solutionswere mixed so that molar ratio of metal ions is 1:2:3 to obtain a mixedsolution. The mixed solution obtained was placed in a round-bottomflask, and was allowed to react for 12 hours in vacuum with purificationusing a rotary evaporator to obtain a semi-transparent blue gel or sol.

A solution, which was obtained by completely dissolving the gel or solby adding methanol (FIG. 1 f) corresponding to 100 times the weight ofthe gel or sol, was purified in vacuum for 12 hours using a rotaryevaporator to obtain a semi-transparent blue gel or sol.

The gel or sol obtained was dissolved again in methanol (FIG. 1 j), andwas diluted using a measuring flask to obtain coating solution 3A with aconcentration of 2.31M as reduced into the concentration of metal ions.According to our past report, it has been shown that the thickness ofthe film formed by dip coating is approximately doubled by using thecoating solution of the above-mentioned concentration due to increasedviscosity as compared with the film formed by using a coating solutionwith a concentration of 1.52M.

Coating solution 3B was obtained by adding H(CF₂)₈COOH to the coatingsolution 3A in a concentration of 10 wt % as an additive. The coatingsolution 3B was placed in a 100 cc beaker so that the depth of thesolution was about 30 mm, an oriented LaAlO₃ single crystal substratewith both-side polished was dipped into the beaker, and the substratewas withdrawn with a withdrawal speed of 50 mm/sec 1 minute after thedipping to obtain a double-sided gel film G3bw50.

The gel film G3bw50 was placed in a calcining furnace, and organicsubstances were decomposed by heating in a moist oxygen atmosphereaccording to the temperature profile shown in FIG. 3 to obtain acalcined film C3bw50 made of a semi-transparent dark brown metaloxy-fluoride.

FIG. 9 shows the result of measurement of the component distribution inthe direction of the substrate from the surface by SIMS analysis ofC3bw50. A reference film was independently prepared by ion-implantationof fluorine and carbon, and fluorine and carbon in C3bw50 film werequantified with reference to the reference film.

In FIG. 9, the position of zero on the horizontal axis denotes thesurface of the film. The content of carbon is higher on the surface ofthe film than the inside of the film, because carbon dioxide in the airis adsorbed on the surface of the film. The content of carbon isgradually decreased at a depth of 2.9 μm or more that is considered tobe the interface between the calcined film and the single crystalsubstrate. The rate of decrease in carbon content is more moderate inFIG. 9 than in FIGS. 5 and 6. This is because there are portions thatarrive at the substrate and do not arrive at the substrate at a depth of2.9 μm since the C3bw50 film is thick and has large surface roughness.Since the composition of the portion that arrives at the substrate islargely different from the composition of the portion that does notarrive at the substrate, reliability of the data decreases at athickness of about 10% of the thickness from the interface between thefilm and substrate. Since the area where an excavated surface by SIMSanalysis arrives at the surface of the substrate increases with time inthis film, the amount of residual carbon as well as the amount ofresidual fluorine are gradually decreased at the portion correspondingto the inside of the substrate.

FIG. 9 shows that the amounts of oxygen, fluorine and copper do notrapidly change at a depth of 200 to 2,610 nm except the portion 200 nmfrom the surface of the film and the portion (290 nm) corresponding to10% of the thickness from the interface between the film and substrate.This shows that a calcined film having a quite large thickness andhaving no discontinuous planes may be obtained. Such calcined film hasbeen considered to be impossible to form by the TFA-MOD method.

Example 4

Sm(OCOCH₃)₃ hydrate was dissolved in ion-exchange water, and mixed withan equimolar amount of CF₃CF₂COOH for reaction with stirring. Thissolution was placed in a round-bottom flask, and was allowed to reactand to be purified for 12 hours in vacuum using a rotary evaporator toobtain a yellow powder. Solution 4PSSm (pre-solution Sm) was obtained bydissolving the powder in methanol.

After obtaining a pale purple powder from Nd(OCOCH₃)₃ hydrate, thispowder was similarly dissolved in methanol to obtain solution 4PSNd.

La(OCOCH₃)₃, Nd(OCOCH₃)₃ and Sm(OCOCH₃)₃ were mixed in a proportion of1:2:7. The mixture was dissolved in ion-exchange water, and was mixedwith an equimolar amount of CF₃CF₂COOH for reaction with stirring. Thissolution was placed in a round bottom flask, and was allowed to reactand to be purified for 12 hours in vacuum using a rotary evaporator toobtain a yellow powder. 4PSMix was obtained by dissolving the powder inmethanol.

Each of 4PSSm, 4PSNd and 4PSMix was completely dissolved in methanol(FIG. 1 f) corresponding to 100 times by weight. Each solution waspurified for 12 hours in vacuum using a rotary evaporator, and yellow,pale purple and yellow powders corresponding to 4PSSm, 4PSNd and 4PSMix,respectively, were obtained. These powders were dissolved in methanol toobtain solutions 4SSm, 4SNd and 4SMix, respectively.

On the other hand, each powder of hydrates of Ba(OCOCH₃)₂ andCu(OCOCH₃)₂ was dissolved in ion-exchange water, and was mixed with anequimolar amount of CF₃COOH for reaction with stirring. These solutionswere mixed so that a molar ratio of metal ions was 2:3, to obtain amixed solution. The mixed solution obtained was placed in a round-bottomflask, and was allowed to react and to be purified in vacuum for 12hours using a rotary evaporator to obtain a semi-transparent blue gel orsol.

The gel or sol obtained was completely dissolved by adding methanol(FIG. 1 f) corresponding to about 100 times the weight of the gel orsol. The thus obtained solution was purified in vacuum for 12 hoursusing a rotary evaporator to obtain a semi-transparent blue sol or gel.The gel or sol obtained was dissolved in methanol (FIG. 1 j), anddiluted using a measuring flask to obtain solution 4BaCu.

The solution 4BaCu was mixed with each of the solutions 4SSm, 4SNd and4SMix, and coating solutions 4Sm, 4Nd and 4Mix were prepared so that theratio of lanthanoid metal M (total molar number):Ba:Cu was 1:2:3.

An additive H(CF₂)₈COOH (10 wt % each) was added to the coatingsolutions 4Sm, 4Nd and 4Mix to obtain coating solutions 4SmT, 4NdT and4MixT, respectively. The coating solutions 4SmT, 4NdT and 4MixT wereplaced in respective 100 cc beakers at a depth of about 30 mm, anoriented LaAlO₃ single crystal substrate with both-side polished wasdipped in each solution, and the substrate was withdrawn at a withdrawalspeed of 50 mm/sec on one minute after the dipping. Two samples each ofdouble-sided gel films G4Smw50, G4Ndw50 and G4Mixw50 were obtained underthe same condition.

Each of the gel films G4Smw50, G4Ndw50 and G4Mixw50 was placed in acalcining furnace, and organic substances were decomposed by heating ina moist oxygen atmosphere according to the temperature profile shown inFIG. 3 to obtain a calcined film made of semi-transparent dark brownmetal oxy-fluoride. These calcined films are referred to as C4Smw50,C4Ndw50 and C4Mixw50.

Compositional distribution was measured in the direction from thesurface to the substrate by SIMS analysis with respect to each ofC4Smw50, C4Ndw50 and C4Mixw50. The results show that, when the film wasdivided in plural regions for every depth of 20 nm from the surface ofthe film, the atomic ratio of copper, fluorine, oxygen or carbon betweentwo adjacent regions was in the range of ⅕ to 5 times.

Each of C4Smw50, C4Ndw50 and C4Mixw50 was placed in a firing furnace,and was fired and annealed in oxygen according to the temperatureprofile shown in FIG. 4 to obtain superconductors F4Smw50, F4Ndw50 andF4Mixw50. The oxygen partial pressures for firing the calcined filmsC4Smw50, C4Ndw50 and C4Mixw50 were 20, 5 and 10 ppm, respectively. Thetemperature for starting oxygen-annealing was 350° C. for all the films,and the temperature was maintained for 4 hours.

Superconducting properties Jc and Tc of the superconducting filmobtained were measured by an inductive method in a self magnetic fieldin liquid nitrogen with CryoScan (manufactured by THEVA Co.). Thethickness of the film was measured by induced coupled plasma emissionspectrometry (ICP) after the measurement by the inductive method. Theresults are shown in Table 1. It was confirmed from Table 1 that thethickness may be increased in the superconductor having high Tc whilehigh Jc is maintained as in the yttrium-based superconductor.

TABLE 1 Jc (77K, 0T) Tc Thickness [MA/cm²] [K] [nm] F4Smw50 4.2 94.0 520F4Ndw50 3.1 93.6 530 F4Mixw50 2.7 93.7 510

Jc values were 4.2 MA/cm² (77K, 0T), 3.1 MA/cm² (77K, 0T) and 2.7 MA/cm²(77K, 0T), Tc values were 94.0K, 93.6K and 93.7K, and thicknesses were520, 530 and 510 nm.

Example 5

Each powder of hydrates of Y(OCOCH₃)₃, Ba(OCOCH₃)₂ and Cu(OCOCH₃)₂ wasdissolved in ion-exchange water, and was mixed with an equimolar amountof CF₃COOH for reaction with stirring. These solutions were mixed sothat the molar ratio of metal ions is 1:2:3, to obtain a mixed solution.The mixed solution obtained was placed in a round-bottom flask, and wasallowed to react and to be purified for 12 hours in vacuum using arotary evaporator to obtain a semi-transparent blue gel or sol.

The gel or sol obtained was completely dissolved by adding methanolcorresponding to 100 times the weight of the sol or gel (FIG. 1 f), andthe obtained solution was reacted and purified for 12 hours in vacuumusing a rotary evaporator to obtain a semi-transparent blue sol or gel.

The gel or sol obtained was dissolved in methanol (FIG. 1 j), andcoating solution 5A with a concentration of 1.52M as reduced to metalions was obtained by dilution using a measuring flask.

Solutions 5B01 to 5B37 were prepared by adding any one of the followingcompounds 01 to 37 to the coating solution 5A as an additive in aconcentration of 10 wt %.

-   [01] F(CF₂)₃OCF(CF₃)CH₂OH-   [02] F(CH₂)₈CH₂CH₂OH-   [03] F(CH₂)₁₀CH₂CH₂OH-   [04] C₃F₇OCF(CF₃)CF₂OCF(CF₃)CH₂OH-   [05] (CF₃)₂CF(CF₂)₄CH₂CH₂OH-   [06] (CF₃)₂CF(CF₂)₆CH₂CH₂OH-   [07] H(CF₂)₆CH₂OH-   [08] H(CF₂)₈CH₂OH-   [09] F(CF₂)₄COOH-   [10] F(CF₂)₅COOH-   [11] F(CF₂)₆COOH-   [12] F(CF₂)₇COOH-   [13] F(CF₂)₈COOH-   [14] F(CF₂)₉COOH-   [15] F(CF₂)₁₀COOH-   [16] F(CF₂)₃O[CF(CF₃)CF₂O]₂CF(CF₃)COF-   [17] F(CF₂)₃O[CF(CF₃)CF₂O]₃CF(CF₃)COF-   [18] H(CF₂)₄COOH-   [19] H(CF₂)₆COOH-   [20] H(CF₂)₈COOH-   [21] HOOC(CF₂)₃COOH-   [22] HOOC(CF₂)₄COOH-   [23] HOOC(CF₂)₆COOH-   [24] HOOC(CF₂)₇COOH-   [25] (CF₃)₂C(CH₃)COOH-   [26] (CF₃)₂C(CH₃)COF-   [27] hexafluoroepoxy propane-   [28] 3-perfluorohexyl-1,2-epoxy propane-   [29] 3-perfluorooctyl-1,2-epoxy propane-   [30] 3-perfluorodecyl-1,2-epoxy propane-   [31] 3-(perfluoro-5-methylhexyl)-1,2-epoxy propane-   [32] 3-(perfluoro-7-methylhexyl)-1,2-epoxy propane-   [33] CF₃CH═CF₂-   [34] F(CF₂)₄CH═CH₂-   [35] F(CF₂)₆CH═CH₂-   [36] F(CF₂)₈CH═CH₂-   [37] F(CF₂)₁₀CH═CH₂

Each of the coating solutions 5B01 to 5B37 was placed in a 100 cc beakerat a depth of 30 mm, and an oriented LaAlO₃ single crystal substratewith both-side polished was dipped in each solution, and the substratewas withdrawn at a withdrawal speed of 50 mm/sec 1 minute after thedipping to obtain gel films 5G01w50 to 5G37w50.

Each of the gel films 5G01w50 to 5G37w50 was placed in a calciningfurnace, and organic substances were decomposed by heating in a moistoxygen atmosphere according to the temperature profile shown in FIG. 3to obtain a calcined film made of a semi-transparent dark brown metaloxy-fluoride.

The heat treatment temperature differs depending on the additives: thegel films were kept in the range of 170 to 220° C. when the additives[09] to [11], [18], [19] and [33] to [37] were used; in the range of 230to 280° C. when the additives [01] to [08] and [21] to [24] were used;and in the range of 200 to 250° C. when the other additives were used.The calcined films are referred to as 5C01w50 to 5C37w50, respectively,hereinafter.

Each of the calcined films 5C01w50 to 5C37w50 was placed in a firingfurnace, and was fired and annealed in oxygen according to thetemperature profile shown in FIG. 4 to obtain superconductors 5F01w50 to5F37w50.

Superconducting properties of each superconducting film obtained weremeasured by an inductive method in a self magnetic field in liquidnitrogen with CryoScan (manufactured by THEVA Co.). The thickness of thefilm was measured by destructive analysis by induced coupled plasmaemission spectrometry (ICP) after the measurement by the inductivemethod. The superconducting films 5F01w50 to 5F37w50 had a thickness inthe range of 400 to 650 nm, and a Jc value of 0.0 to 5.6 MA/cm² (77K,0T).

In this case, the Jc value is related to the ratio (mol %) of atomicnumbers of (fluorine)/(fluorine+hydrogen) in the additive molecule. Thisratio is referred to as R_(F). Table 2 shows the relation between theR_(F) value of the additive and the Jc value of the superconducting filmobtained. The data in Table 2 are summarized in FIG. 10.

TABLE 2 Jc (77K, 0T) Fluorine Hydrogen R_(F) [MA/cm²] CF₃CF₂(CH₂)₆OH 513 27.8% 0.0 F(CF₂)₄(CH₂)₆OH 9 13 40.9% 0.0 CHF₂CF₂CH₂OH 4 4 50.0% 0.0CF₃CHFCF₂CH₂OH 6 4 60.0% 0.8 F(CF₂)₈(CH₂)₃OH 17 7 70.8% 1.6 H(CF₂)₆CH₂OH12 4 75.0% 4.1 H(CF₂)₈CH₂OH 16 4 80.0% 4.7 C₃F₇OCF(CF₃)CF₂OCF(CF₃)CH₂OH17 3 85.0% 5.1 F(CF₂)₄COOH 9 1 90.0% 5.3 F(CF₂)₁₀COOH 21 1 95.5% 5.6F(CF₂)₃O[CF(CF₃)CF_(2O)]₂CF(CF₃)COF 24 0 100.0% 5.8

Table 2 and FIG. 10 show the following facts. The Jc value rapidlydecreases when R_(F) is less than 75%. Since additives containing alarge amount of hydrogen remain in the film even after organic chainsare broken as described above, oxygen in other oxides tends to bereplaced with fluorine to increase the amount of residual fluorine toconsequently degrade superconducting properties. Conversely, the effectof additives containing a small amount of hydrogen seems to give littleinfluence since compounds of fluorides and hydrogen fluoride are formedand are dissipated in air. R_(F) is preferably 75% or more for obtaininga superconductor that exhibits sufficient superconducting properties.Additives with R_(F) of 90% or more are particularly preferable sincesuperconducting films with Jc of 5.0 MA/cm² (77K, 0T) or more can beobtained. However, cracks are liable to occur when R_(F) exceeds 96%.This may be supposed that, since additives containing too much fluorinedo not form sufficient hydrogen bonds in the film, drying stressresistance of the film during decomposition of trifluoroacetate salt isso weakened that cracks are readily produced.

Compositional distribution in the direction from the surface of the filmto the substrate was measured by SIMS analysis for each of thesuperconducting films 5F01w50 to 5F37w50. The results show that, whenthe film was divided into plural regions for every 10 nm from the filmsurface in the direction of thickness, except a region 100 nm or lessfrom the surface of the film and a region corresponding to 10% of thethickness from the interface between the film and substrate, atomicratios (average) of oxygen, fluorine and copper, respectively, betweenadjacent two regions were in the range of ⅕ to 5 times, and theconcentration changes of these elements were continuous.

Example 6

Powders of hydrates of Y(OCOCH₃)₃, Ba(OCOCH₃)₂ and Cu(OCOCH₃)₂ weredissolved in ion-exchange water, and each solution was mixed with anequimolar amount of CF₃COOH for reaction with stirring. These solutionswere mixed so that molar ratio of metal ions was 1:2:3, to obtain amixed solution. The mixed solution obtained was placed in a round-bottomflask, and was allowed to react for 12 hours in vacuum with purificationusing a rotary evaporator to obtain a semi-transparent blue gel or sol.

A solution, which was obtained by completely dissolving the gel or solby adding methanol (FIG. 1 f) corresponding to 100 times the weight ofthe gel or sol, was purified in vacuum for 12 hours using a rotaryevaporator to obtain a semi-transparent blue gel or sol.

The gel or sol obtained was dissolved again in methanol (FIG. 1 j), andwas diluted using a measuring flask to obtain coating solution 6A with aconcentration of 1.52M as reduced into the concentration of metal ions.

The following organic compounds were provided as additives:

-   [07] H(CF₂)₆CH₂OH-   [08] H(CF₂)₈CH₂OH-   [09] F(CF₂)₄COOH-   [10] F(CF₂)₅COOH-   [11] F(CF₂)₆COOH-   [13] F(CF₂)₈COOH-   [15] F(CF₂)₁₀COOH-   [18] H(CF₂)₄COOH-   [19] H(CF₂)₆COOH-   [20] H(CF₂)₈COOH

Coating solutions were obtained by adding 5 wt % each of two additivesto the coating solution 6A. For example, a coating solution prepared bymixing 5 wt % each of the additives [07] and [09] is referred to ascoating solution S0709. The decomposition temperature range was widenedby mixing two additives in this manner, and stability against heattreatment tends to be increased.

Eleven coating solutions S0709, S0809, S0910, S0911, S0913, S0915,S1118, S1119, S1120, S0918 and S1318 were prepared in this example.

Each of the coating solutions was placed in a 100 cc beaker at a depthof 30 mm, an oriented LaAiO₃ single crystal substrate with both-sidepolished was dipped in each solution, and the substrate was withdrawn ata withdrawal speed of 50 mm/sec on one minute after the dipping toobtain gel films G0709, G0809, G0910, G0911, G0913, G0915, G1118, G1119,G1120, G0918 and G1318.

Each of the gel films G0709, G0809, G0910, G0911, G0913, G0915, G1118,G1119, G1120, G0918 and G1318 was placed in a calcining furnace, andorganic substances were decomposed in a moist oxygen atmosphereaccording to the temperature profile shown in FIG. 3 to obtain acalcined film made of semi-transparent dark brown metal oxy-fluoride.Each gel film was kept in a selected heat temperature range of 50° C.between 170° C. and 240° C., depending on the additive, for a longperiod. For example, G0709, G0809, G0910, G0911, G0913 and G0915 weremaintained at 180 to 230° C., G1119 and G1120 were maintained at 190 to240° C., and the others were maintained at 170 to 220° C. These calcinedfilms are referred to as C0709, C0809, C0910, C0911, C0913, C0915,C1118, C1119, C1120, C0918 and C1318.

Each of these calcined films was placed in a firing furnace, and wasfired and annealed in oxygen according to the temperature profile shownin FIG. 4 to obtain superconducting films F0709, F0809, F0910, F0911,F0913, F0915, F1118, F1119, F1120, F0918 and F1318.

Superconducting properties of each superconducting film were measured byan inductive method in a self magnetic field in liquid nitrogen withCryoScan (manufactured by THEVA Co.). The thickness of the film wasmeasured by destructive analysis by induced coupled plasma emissionspectrometry (ICP) after the measurement by the inductive method. Theresults are shown in Table 3.

TABLE 3 Jc (77K, 0T) Thickness [MA/cm²] [nm] F0709 4.7 410 F0809 4.9 440F0910 4.8 460 F0911 5.1 470 F0913 5.3 480 F0915 5.4 500 F1118 4.8 480F1119 5.1 500 F1120 5.3 520 F0918 4.4 420 F1318 4.8 470

The compositional distribution was measured by SIMS analysis in thedirection from the surface to the substrate with respect to each of thesuperconducting films F0709, F0809, F0910, F0911, F0913, F0915, F1118,F1119, F1120, F0918 and F1318. The results show that, when the film wasdivided into plural regions for every 10 nm from the film surface in thedirection of thickness, except a region 100 nm or less from the surfaceof the film and a region corresponding to 10% of the thickness from theinterface between the film and substrate, atomic ratios (average) ofoxygen, fluorine and copper, respectively, between adjacent two regionswere in the range of ⅕ to 5 times, and the concentration changes ofthese elements were continuous. It is also shown that the decompositiontemperature slightly decreases and the decomposition temperature rangetends to be broad when two kinds of additives were mixed and added, andexcellent superconducting properties are obtained with a thicksuperconducting film.

Example 7

Powders of hydrates of Y(OCOCH₃)₃, Ba(OCOCH₃)₂ and Cu(OCOCH₃)₂ weredissolved in ion-exchange water, and each solution was mixed with anequimolar amount of CF₃COOH for reaction with stirring. These solutionswere mixed so that molar ratio of metal ions was 1:2:3, to obtain amixed solution. The mixed solution obtained was placed in a round-bottomflask, and was allowed to react for 12 hours in vacuum with purificationusing a rotary evaporator to obtain a semi-transparent blue gel or sol.

A solution, which was obtained by completely dissolving the gel or solby adding methanol (FIG. 1 f) corresponding to 100 times the weight ofthe gel or sol, was purified in vacuum for 12 hours using a rotaryevaporator to obtain a semi-transparent blue gel or sol.

The gel or sol obtained was dissolved again in methanol (FIG. 1 j), andwas diluted using a measuring flask to obtain coating solution 7A with aconcentration of 1.52M as reduced into the concentration of metal ions.

The following organic compounds were provided as additives:

-   [01] F(CF₂)₄COOH-   [02] F(CF₂)₅COOH-   [03] F(CF₂)₆COOH-   [04] F(CF₂)₇COOH-   [05] F(CF₂)₈COOH-   [06] F(CF₂)₉COOH-   [07] F(CF₂)₁₀COOH-   [08] HOOC(CF₂)₄COOH-   [09] HOOC(CF₂)₆COOH-   [10] HOOC(CF₂)₇COOH-   [11] (CF₃)₂C(CH₃)COOH-   [12] (CF₃)₂C(CH₃)COF

A coating solution was obtained by adding 10 additives of theabove-mentioned additives in an amount of 1 wt % each (10 wt % in total)to the coating solution 7A. The mixed additives were a mixture of [01]to [10]; a mixture of [01] to [09] and [11]; and a mixture of [01] to[09] and [12]. The coating solutions obtained are referred to as coatingsolutions 7X, 7Y and 7Z.

Each of the coating solutions was placed in a 100 cc beaker at a depthof 30 mm, an oriented LaAlO₃ single crystal substrate with both-sidepolished was dipped in each solution, and the substrate was withdrawn ata withdrawal speed of 50 mm/sec on one minute after the dipping toobtain gel films G7X, G7Y and G7Z.

Each of the gel films G7X, G7Y and G7Z was placed in a calciningfurnace, and organic substances were decomposed in a moist oxygenatmosphere according to the temperature profile shown in FIG. 3 toobtain a calcined film made of semi-transparent dark brown metaloxy-fluoride. The heat treatment temperature was in the range of 180 to230° C., and the temperature was maintained for a long period. Thesecalcined films are referred to as C7X, C7Y and C7Z. It was confirmedthat the film may be stably formed up to the periphery of the film whenplural additives were used by mixing. The optimum decompositiontemperature was decreased by 20° C.

Each of the calcined films C7X, C7Y and C7Z was placed in a firingfurnace, and was fired and annealed in oxygen according to thetemperature profile shown in FIG. 4 to obtain superconducting films F7X,F7Y and F7Z.

Superconducting properties of each superconducting film were measured byan inductive method in a self magnetic field in liquid nitrogen withCryoScan (manufactured by THEVA Co.). The thickness of the film wasmeasured by destructive analysis by induced coupled plasma emissionspectrometry (ICP) after the measurement by the inductive method. Theresults are shown in Table 4.

TABLE 4 Jc (77K, 0T) Thickness [MA/cm²] [nm] F7X 5.6 510 F7Y 5.8 520 F7Z6.0 510

The compositional distribution was measured by SIMS analysis in thedirection from the surface to the substrate with respect to each of thesuperconducting films F7X, F7Y and F7Z. The results show that, when thefilm was divided into plural regions for every 10 nm from the filmsurface in the direction of thickness, except a region 100 nm or lessfrom the surface of the film and a region corresponding to 10% of thethickness from the interface between the film and substrate, atomicratios (average) of oxygen, fluorine and copper, respectively, betweenadjacent two regions were in the range of ⅕ to 5 times, and theconcentration changes of these elements were continuous.

Example 8

Powders of hydrates of Y(OCOCH₃)₃, Ba(OCOCH₃)₂ and Cu(OCOCH₃)₂ weredissolved in ion-exchange water, and each solution was mixed with anequimolar amount of CF₃COOH for reaction with stirring. These solutionswere mixed so that molar ratio of metal ions was 1:2:3, to obtain amixed solution. The mixed solution obtained was placed in a round-bottomflask, and was allowed to react for 12 hours in vacuum with purificationusing a rotary evaporator to obtain a semi-transparent blue gel or sol.

A solution, which was obtained by completely dissolving the gel or solby adding methanol (FIG. 1 f) corresponding to 100 times the weight ofthe gel or sol, was purified in vacuum for 12 hours using a rotaryevaporator to obtain a semi-transparent blue gel or sol.

The gel or sol obtained was dissolved again in methanol (FIG. 1 j), andwas diluted using a measuring flask to obtain coating solution 8A with aconcentration of 1.52M as reduced into the concentration of metal ions.

Ten wt % of F(CF₂)₄COOH or H(CF₂)₄COOH was added to the coating solution8A as an additive to obtain coating solutions 8F and 8H.

Each of the coating solutions was placed in a 100 cc beaker at a depthof 30 mm, an oriented LaAlO₃ single crystal substrate with both-sidepolished was dipped in each solution, and the substrate was withdrawn ata withdrawal speed of 5 mm/sec on one minute after the dipping to obtaingel films G8F and G8H. The withdrawal speed was reduced because, whenthe withdrawal speed was increased for increasing the thickness of thefilm, a good film could not be formed in the G8H sample. Under suchcircumstances, the slow withdrawal speed was selected for comparing theeffects of both additives.

Each of the gel films G8F and G8H was placed in a calcining furnace, andorganic substances were decomposed by heating in a moist oxygenatmosphere according to the temperature profile shown in FIG. 3 toobtain a calcined film made of semi-transparent dark brown metaloxy-fluoride. These calcined films are referred to as C8F and C8H.

Each of the calcined films C8F and C8H was placed in a firing furnace,and was fired and annealed in oxygen according to the temperatureprofile shown in FIG. 4 to obtain superconducting films F8F and F8H.

Superconducting properties of each superconducting film obtained weremeasured by an inductive method in a self magnetic field in liquidnitrogen with CryoScan (manufactured by THEVA Co.). The thickness of thefilm was measured by destructive analysis by induced coupled plasmaemission spectrometry (ICP) after the measurement by the inductivemethod. Jc values (77K, 0T) of the superconducting films F8F and F8Hwere 6.2 and 0.0 MA/cm², respectively, with the same thickness of 170nm. The results show that the film F8H was not a superconductor.

Fluorine distribution in F8F and F8H in the direction of the substratefrom the surface was measured by SIMS analysis. The results are shown inFIG. 11. The results of a physical vapor deposition film (deposited) anda physical vapor deposition film with fluorine ion-implantation(implanted) are also shown in FIG. 11.

The amount of fluorine gradually decreased from the surface of the filmin F8F, and the concentration of fluorine tended to be about 10 times ofthe background level inside the film. It has been found that, whenF(CF₂)₄COOH is used as the additive, the amount of residual fluorine isapproximately the same as in FIG. 11 even by increasing the thickness ofthe film by increasing the withdrawal speed. This shows that the amountof fluorine may be effectively reduced in the superconductor by usingF(CF₂)₄COOH as the additive.

It is also shown that fluorine remains in F8H in an amount about 10times of F8F. The amount of elimination of fluorine is quite small evenby forming the quasi-liquid phase network during the firing process whenH(CH₂)₄COOH is used as the additive, and the obtained superconductingfilm contains a large amount of fluorine. Since the perovskite structureof the superconductor is disrupted by forming BaF₂ throughrecrystallization when the temperature decreases after the firingprocess in the superconducting film containing fluorine, superconductingproperties are significantly degraded to 1/10 or less of the filmcontaining a small amount of fluorine.

The amount of residual fluorine was 2 to 3 times of the amount in FIG.11 assuming that the thickness of F8H is 300 nm that is close to thecritical thickness. This may be ascribed to obstruction of dissipationof fluorine compounds by hydrogen atoms as a result of increasedthickness. Fluorine located far from the surface tends to remain, andthis gives good elucidation that superconducting properties are degradedwhen the amount of residual fluorine increases.

It may be reasonably presumed that a high-purity solution containing noacetate salts having many hydrogen atoms is indispensable for obtainingthick superconductor with few fluorine residues. This is because theresidual amount of fluorine increases when acetate salts having manyhydrogen atoms are co-existent. This tendency becomes remarkable as thethickness of the film increases. When organic compounds having manyhydrogen atoms remain in an initial solution, the amount of residualfluorine in the superconducting film increases according to filmthickness, and superconducting properties are degraded.

Example 9

Powders of hydrates of Y(OCOCH₃)₃, Ba(OCOCH₃)₂ and Cu(OCOCH₃)₂ weredissolved in ion-exchange water, and each solution was mixed with anequimolar amount of CF₃COOH for reaction with stirring. These solutionswere mixed so that a molar ratio of metal ions was 1:2:3, to obtain amixed solution. The mixed solution obtained was placed in a round-bottomflask, and was allowed to react for 12 hours in vacuum with purificationusing a rotary evaporator to obtain a semi-transparent blue gel or sol.

A solution, which was obtained by completely dissolving the gel or solby adding methanol (FIG. 1 f) corresponding to 100 times the weight ofthe gel or sol, was purified in vacuum for 12 hours using a rotaryevaporator to obtain a semi-transparent blue gel or sol.

The gel or sol obtained was dissolved again in methanol (FIG. 1 j), andwas diluted using a measuring flask to obtain coating solutions 9A, 9Band 9C with concentrations of 1.52M, 2.31M and 2.78M, respectively, asreduced into the concentration of metal ions.

H(CF₂)₄COOH as an additive was added to each of the coating solutions9A, 9B and 9C in a proportion of 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 6.0,8.0, 10, 15, 20, 25 or 30 wt % to obtain coating solutions. A coatingsolution containing 1.5 wt % of the additive added to the solution 9C isreferred to, for example, as 9C1.5.

Each of the coating solutions was placed in a 100 cc beaker at a depthof 30 mm, an oriented LaAlO₃ single crystal substrate with both-sidepolished was dipped in each solution, and the substrate was withdrawn ata withdrawal speed of 5 to 50 mm/sec on one minute after the dipping toobtain gel films. The gel film obtained by withdrawing the substrate ata withdrawal speed of 20 mm/sec from the solution 9C1.5 is referred to,for example, as G9C1.5w20.

Each of gel films G8F and G8H was placed in a calcining furnace, andorganic substances were decomposed by heating in a moist oxygenatmosphere according to the temperature profile shown in FIG. 3 toobtain a calcined film made of semi-transparent dark brown metaloxy-fluoride. The calcined film manufactured from the gel film G9C1.5w20is referred to, for example, as C9C1.5w20.

Each of the calcined films was placed in a firing furnace, and was firedand annealed in oxygen according to the temperature profile shown inFIG. 4 to obtain a superconducting film. The superconducting filmobtained from the calcined film C9C1.5w20 is referred to, for example,as F9C1.5w20.

Occurrence of cracks was investigated for each calcined film. When theamount of the additive was 2.0 wt % or less, occurrence of cracks wasconfirmed in the superconducting films (for example C9A2.0w15, C9B2.0w15and C9C2.0w15) having a thickness of about 350 nm and manufactured at awithdrawal speed of 15 mm/sec. Similar tendency was shown when theamount of the additive was 2.0 wt % or less irrespective of theconcentration of the solution. On the other hand, the effect ofincreasing the thickness of the film was supposed to be exhibited whenthe amount of the additive was 2.5 wt % or more.

Superconducting properties of the superconducting film (for exampleC9A25w20) with amount of additive over 20 wt % were poor, although nocracks were produced. Superconducting properties are degraded becausethe ratio of total amount of additive material is relative large in caseof the salts. This tendency was similar in the range where the amount ofthe additive exceeded 20 wt % irrespective of the concentration of thesolution.

These results suggest that the ratio between trifluoroacetate salt andadditive in the gel film is important for increasing the thickness ofthe superconducting film.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. An oxide superconductor film formed on a substrate, comprising: anoxide containing at least one metal M selected from the group consistingof yttrium and lanthanoid metals, provided that cerium, praseodymium,and promethium are excluded, and barium and copper, wherein the film hasan average thickness of 350 nm or more, an average amount of residualcarbon of 3×10¹⁹ atoms/cc or more, and an amount of residual fluorine ina range of 5×10¹⁷ to 1×10¹⁹ atoms/cc, and wherein, when divided the filminto a plurality of regions from a surface of the film or from aninterface between the film and the substrate, each region having athickness of 10 nm, atomic ratios of copper, fluorine, oxygen and carbonbetween two adjacent regions are in a range of ⅕ times to 5 times. 2.The oxide superconductor film according to claim 1, wherein the film hasno cracks with a width of 0.1 mm or more and a length of 1 mm or more ina region except a region within 2 mm from an edge of the substrate. 3.The oxide superconductor film according to claim 1, wherein an atomicratio of metal M, barium and copper is about 1:2:3.
 4. The oxidesuperconductor film according to claim 1, wherein the film has anaverage thickness of 600 nm or more.
 5. The oxide superconductor filmaccording to claim 4, wherein the film has an average thickness of 1,250nm or more.
 6. The oxide superconductor film according to claim 1,wherein, when divided the film into a plurality of regions from thesurface of the film or from the interface between the film and thesubstrate, each region having a thickness of 10 nm, atomic ratios ofcopper, fluorine, oxygen and carbon between two adjacent regions are ina range of ⅓ times to 3 times.